Over 170 Honeywell UOP Separex™ membrane systems have been installed in the world for gas separation applications such as for the removal of acid gases from natural gas, in enhanced oil recovery, and hydrogen purification. Two new Separex™ membranes (Flux+ and Select) have been commercialized recently by Honeywell UOP, Des Plaines, Ill. for carbon dioxide (CO2) removal from natural gas. These Separex™ spiral wound membrane systems currently hold the membrane market leadership for natural gas upgrading. These membranes prepared from glassy polymers, however, do not have outstanding performance for organic vapor separations such as for olefin recovery, liquefied petroleum gas (LPG) recovery, fuel gas conditioning, natural gas dew point control, nitrogen removal from natural gas, etc.
Polymeric membrane materials have been found to be of use in gas separations. Numerous research articles and patents describe glassy polymeric membrane materials (e.g., polyimides, polysulfones, polycarbonates, polyamides, polyarylates, polypyrrolones) with desirable gas separation properties, particularly for use in oxygen/nitrogen separation (see, for example, U.S. Pat. No. 6,932,589). The polymeric membrane materials are typically used in processes in which a feed gas mixture contacts the upstream side of the membrane, resulting in a permeate mixture on the downstream side of the membrane with a greater mole fraction of one of the components than the composition of the original feed gas mixture. A pressure differential is maintained between the upstream and downstream sides, providing the driving force for permeation. The downstream side can be maintained as a vacuum, or at any pressure below the upstream pressure.
The separation of a polymeric membrane is based on a solution-diffusion mechanism. This mechanism involves molecular-scale interactions of the permeating gas with the polymer. The mechanism assumes that in a membrane having two opposing surfaces, each component is sorbed by the membrane at one surface, transported by a gas concentration gradient, and desorbed at the opposing surface. According to this solution-diffusion model, the membrane performance in separating a given pair of gases (e.g., CO2/CH4, O2/N2, H2/CH4) is determined by two parameters: the permeability coefficient (abbreviated hereinafter as permeability or PA) and the selectivity (αA/B). The PA is the product of the gas flux and the selective skin layer thickness of the membrane, divided by the pressure difference across the membrane. The αA/B is the ratio of the permeability coefficients of the two gases (αA/B=PA/PB) where PA is the permeability of the more permeable gas and PB is the permeability of the less permeable gas. Gases can have high permeability coefficients because of a high solubility coefficient, a high diffusion coefficient, or because both coefficients are high. In general, the diffusion coefficient decreases while the solubility coefficient increases with an increase in the molecular size of the gas. In high performance polymer membranes, both high permeability and selectivity are desirable because higher permeability decreases the size of the membrane area required to treat a given volume of gas, thereby decreasing capital cost of membrane units, and because higher selectivity results in a higher purity product gas.
The relative ability of a membrane to achieve the desired separation is referred to as the separation factor or selectivity for the given mixture. There are, however, several other obstacles to use a particular polymer to achieve a particular separation under any sort of large scale or commercial conditions. One such obstacle is permeation rate or flux. One of the components to be separated must have a sufficiently high permeation rate at the preferred conditions or extraordinarily large membrane surface areas are required to allow separation of large amounts of material. Therefore, commercially available glassy polymeric membranes, such as CA, polyimide, and polysulfone membranes formed by phase inversion and solvent exchange methods have an asymmetric integrally skinned membrane structure. See U.S. Pat. No. 3,133,132. Such membranes are characterized by a thin, dense, selectively semipermeable surface “skin” and a less dense void-containing (or porous), non-selective support region, with pore sizes ranging from large in the support region to very small proximate to the “skin”. Plasticization occurs when one or more of the components of the mixture act as a solvent in the polymer often causing it to swell and lose its membrane properties. It has been found that glassy polymers such as cellulose acetate and polyimides which have particularly good separation factors for separation of mixtures comprising carbon dioxide and methane are prone to plasticization over time thus resulting in decreasing performance of these membranes.
Natural gas often contains substantial amounts of heavy hydrocarbons and water, either as an entrained liquid, or in vapor form, which may lead to condensation within membrane modules. The gas separation capabilities of glassy polymeric membranes are affected when contacting with liquids including water and aromatic hydrocarbons such as benzene, toluene, ethylbenzene, and xylene (BTEX). The presence of more than modest levels of liquid BTEX heavy hydrocarbons is potentially damaging to traditional glassy polymeric membrane. Therefore, precautions must be taken to remove the entrained liquid water and heavy hydrocarbons upstream of the glassy polymeric membrane separation steps using expensive membrane pretreatment system. Another issue of glassy polymeric polymer membranes that still needs to be addressed for their use in gas separations in the presence of high concentration of condensable gas or vapor such as CO2 and propylene is the plasticization of the glassy polymer by these condensable gases or vapors that leads to swelling of the membrane as well as a significant increase in the permeance of all components in the feed and a decrease in the selectivity of the membranes.
Some natural gas also contains substantial amount of nitrogen (N2) in additional to the heavy hydrocarbons, water, and acid gases such as CO2 and hydrogen sulfide (H2S). Traditional glassy polymeric membranes are relatively more permeable to N2 than to methane. These membranes, however, have low N2 permeance and low N2/CH4 selectivity of less than 5.
For glassy polymeric gas separation membranes, permeant diffusion coefficient is more important than its solubility coefficient. Therefore, these glassy polymeric gas separation membranes preferentially permeate the smaller, less condensable gases, such as H2 and CH4 over the larger, more condensable gases, such as C3H8 and CO2. On the other hand, in rubbery polymeric membranes such as polydimethylsiloxane membrane, permeant solubility coefficients are much more important than diffusion coefficient. Thus, these rubbery polymeric membranes preferentially permeate the larger, more condensable gases over the smaller, less condensable gases. Polydimethylsiloxane (PDMS) is the most commonly used rubbery membrane material for separation of higher hydrocarbons or methane from permanent gases such as N2 and H2.
Most of the polyolefin such as polypropylene (PP) and polyethylene (PE) manufacturing plants and other polymer such as polyvinyl chloride (PVC) manufacturing plants use a degassing step to remove un-reacted olefins, solvents, and other additives from the raw polyolefin. Nitrogen is normally used as the stripping gas or for the polymer transfer. Disposing of the vent stream in a flare or partial recovery of the valuable olefin or other monomers via a condensing process results in the loss of valuable monomers and undesired emissions of the highly reactive volatile monomers into the air. Typically, the vent stream of the polymer reactor is compressed and then cooled to condense the monomers such as propylene and ethylene from the PP and PE reactors. The gas leaving the condenser still contains a significant amount of the monomers. One application for rubbery polymeric membranes is to recover the valuable monomers such as propylene, ethylene, and vinyl chloride and purify nitrogen for reuse from the vent stream. For olefin splitter overhead applications, the stream leaving the column overhead is primarily olefins, mixed with light gases such as N2 or H2. The membrane can separate the stream into an olefin-enriched stream and a light-gas-enriched stream. The olefin-enriched stream is returned to the distillation column, where the high value olefin is recovered, and the light-gas-enriched stream is vented or flared. The condensation/membrane hybrid process will achieve significantly higher olefin recovery than condensation process alone and also allows olefin recovery at moderate temperatures and pressures than condensation process.
Ethylene recovery during the ethylene oxide (EO) production process to prevent the loss of valuable ethylene feedstock is another potential application of rubbery polymeric membranes. The rubbery polymeric membrane separates ethylene from argon purge gas by permeating ethylene at a much faster rate than argon to generate ethylene-enriched permeate that will be returned to the EO reactor and argon-enriched residue that will be flared.
The rubbery polymeric membrane can also be used for fuel gas conditioning that will reduce heavier hydrocarbons and increase CH4 content (methane number) in the fuel gas which will be used to power upstream oil and gas operations while maintaining the pressure of the tail gas. Glassy polymeric membranes normally have very low methane permeance and also relatively low methane/heavy hydrocarbon selectivities.
Combining the features of a glassy polymeric membrane and a rubbery polymeric membrane into a multiple membrane system provides a system having the advantages of both of the types of membranes. The membranes may be in any order in the system and multiple glassy polymeric membranes and multiple rubbery polymeric membranes may be used.